This bacterium, pathogenic for swine, horses, and cattle, is also known to cause disease in compromised human hosts. When grown on culture media, it produces pale pink colonies that help to presumptively identify it as

• A. Arcanobacterium hemolyticum
• B. Actinomyces naeslundii
• C. Gardnerella vaginalis
• D. Rhodococcus equi

Answer: (D)

Rhodococcus equi is distinguished in the lab by its distinctive pigment production. When cultured on routine media, it forms pale salmon-pink (pale pink) colonies due to carotenoid pigments. That color cue is a helpful presumptive identifier for this organism, especially in the clinical context described, where it is known to infect horses, swine, and cattle and can cause disease in humans with compromised immunity. In addition to the pigment, this organism is a Gram-positive, aerobic coccobacillus and can show weak acid-fast staining with a modified stain, but the pink colonies on culture are the quickest, most practical clue used to recognize it. The other organisms listed do not produce this characteristic pink pigment on standard media, so they are less consistent with the described colony appearance.